1. Field of the Invention
This invention relates to the field of solar energy. More specifically, the invention comprises a solar collector incorporating multiple parabolic troughs and multiple collector pipes running through the troughs, in which the position of the pipes relative to the troughs is varied in order to keep the collector pipes in the focus of the troughs as the sun moves across the sky.
2. Description of the Related Art
Solar energy collecting devices frequently use focusing lenses or reflectors to intensify the energy of the sun. Some collecting devices directly convert the solar energy to electrical energy using a photovoltaic array. Other collecting devices use the solar energy to heat a circulating working fluid. The present invention may be adapted to either type of collecting devices, as well as other types.
FIG. 1 shows an elevation view of a prior art solar collector suitable for heating a circulating working fluid. Parabolic trough 10 receives incoming rays 12. Because the sun may be considered to be an infinite distance away, the incoming rays are effectively parallel. The parabola that is used to define parabolic trough 10 is selected to bring the parallel incoming rays to the same point central focal point 14. Meeting this limitation produces the maximum collection efficiency.
The reflecting trough shown extends for any suitable distance in a direction that is perpendicular to the orientation of the view. For this type of collector, a conductive pipe containing the circulating working fluid is run through central focal point 14, with the pipe running in a direction that is also perpendicular to the view of FIG. 1.
Those skilled in the art will quickly realize that focal point 14 lies along the parabola's axis of symmetry 15, so long as the incoming rays are parallel to the axis of symmetry. Because the reflecting trough actually extends for some distance in a direction that is perpendicular to the view of FIG. 1, axis of symmetry 15 actually defines a “plane of symmetry” that extends along the length of the trough (the plane of symmetry being perpendicular to the orientation of the view).
In FIG. 1, the sun is located directly above the reflector and the rays are coming straight down. Of course, the sun moves across the sky during the course of the day. Parabolic trough 10 must generally be tilted so that the plane of symmetry running through axis of symmetry 15 remains parallel with the incoming rays. This tilting action is generally referred to as an adjustment in “elevation.” It is indicated by the reciprocating arrow labeled tracking pivot 16.
Prior art parabolic trough collectors typically include a suitable tilting mechanism in order to adjust the elevation of the collector. This mechanism regulates the elevation of the collector throughout the course of the day. An azimuth tracking mechanism is also frequently included. Such mechanisms tend to be complex and relatively expensive.
FIG. 5 shows a simplified representation of a prior art solar collector that includes both elevation and azimuth tracking. Three parallel parabolic trough reflectors 10 are contained within housing 46. The parabolic troughs each have a finite length. The focus of each trough is therefore not a single point but rather an axis that runs through the focal point existing at each section taken through the trough. Thus, as seen in the view, the three trough reflectors have three parallel focus axes 44.
In order to keep the housing perfectly perpendicular to the incoming solar rays it must be adjusted in both elevation and azimuth. Elevation adjustment 48 tilts housing 46 as indicated. Azimuth adjustment 50 rotates the housing so that it tracks the sun crossing the sky.
One may simply set the elevation adjustment to match the latitude of the location and gain a good approximation of the optimum elevation through the middle of the day. Azimuth, however, is not so easy to approximate. A simple visualization exercise demonstrates this fact: If one sets the azimuth of a device such as shown in FIG. 5 so that housing 46 directly faces the sun at sunrise, it is immediately apparent that the trough reflectors will not receive any direct rays by sunset. For this reason, a housing that lacks azimuth tracking is most often set so that the azimuth is correct when the sun is directly overhead.
This static approach works fairly well for photovoltaic cells but it does not work well for parabolic trough reflectors. FIG. 2 graphically illustrates the focusing error that occurs in the absence of a tracking mechanism. This figure shows a static parabolic trough 10 after the sun has moved away from its zenith position. Incoming rays 12 are no longer parallel to the parabola's axis of symmetry 15 (and plane of symmetry). The result is that the incoming rays are no longer reflected toward central focal point 14. Instead, they have shifted to the right toward shifted focal zone 42. The shift is primarily in a direction that is perpendicular to the plane of symmetry running through axis of symmetry 15. The term “focal zone” is used because a parabola only brings incoming rays into sharp focus when the rays are parallel to the parabola's axis of symmetry. Once the incoming rays are angularly offset from the axis of symmetry, the parabola is no longer able to create a perfect focus. However, over a reasonable range of angular displacement, the parabola is still able to create a good concentration of solar energy and the region of this concentration is therefore referred to as a “focal zone.”
The reader will thereby appreciate that it is desirable to position a collector pipe within the focal zone even when the focal zone moves away from the trough collector's plane of symmetry. The present invention presents such a solution.